But ISIL has sparked a fierce backlash from both moderate and Islamist brigades because of alleged abuses against civilians and rival armed opposition groups.

Since January 3, a coalition of those rebel groups has been fighting ISIL across areas under opposition control, including Idlib, Aleppo and Raqa provinces.

The fighting has killed more than 1,700 people, according to the Observatory, and has drawn attention and resources away from the fight against the regime in some places.

ISIL grew from the Islamic State of Iraq, a one-time Al Qaeda affiliate in Iraq that expanded into Syria after the conflict there began.

The group sought to merge with the jihadist Al Nusra Front in Syria, but the Front rebuffed the overture and pledged allegiance directly to Al Qaeda chief Ayman Al Zawahiri.

Zawahiri says Al Nusra is Al Qaeda's official branch in Syria, and has distanced his organisation from ISIL, unsuccessfully ordering them to return to Iraq.

Al Nusra has largely stayed out of the fighting against ISIL, but an array of moderate brigades as well as Islamist groups from the Islamic Front -- including Suqour al-Sham -- have been involved in fierce clashes with ISIL.